In many instances, e.g. in the operation of textile machinery, a load driven by an a-c motor of the induction or possibly the synchronous type changes in a not always predictable manner. A rotating spindle carrying a take-up spool or cop, for example, will require a progressively larger torque as the growing body of yarn wound thereon encounters ever-increasing air resistance. On the other hand, an unforeseen occurrence such as the break of a transmission shaft could sharply lower the load seen by a drive motor.
In my commonly assigned U.S. Pat. No. 4,160,940 I have disclosed and claimed a method of and a system for controlling the operation of a squirrel-cage or other induction motor with avoidance of harmful voltage transients and with stabilization against pull-out. The system described in that prior patent includes a frequency changer with cascaded a-c/d-c and d-c/a-c converters forming part of a polyphase power line for the energization of the stator of such a motor whose rotor is coupled with a tachometer or speed sensor feeding back an error signal when the rotor speed deviates from a selected value. A compensatory adjustment of the magnitude of the stator current is carried out in response to that error signal by a control unit including the a-c/d-c converter of the frequency changer; this control unit, however, operates with a certain lag due to the presence of reactances such as a current-stabilizing choke inserted in a direct-current path between the two converters. The error signal is also delivered to a feedback circuit of relatively small time constant controlling a pulse generator which, by acting upon the d-c/a-c converter, modifies the frequency of the alternating voltage supplied to the input of the stator and thus the speed of the rotating electromagnetic field generated therein; this modifies the slip of the rotor relative to that field and with it the torque developed by the motor before any significant change in the stator current. Contrary to an earlier system referred to in that prior patent, the frequency change caused by this fast feedback temporarily reduces the slip to stabilize the motor against pull-out in the event of a load increase. When the slow-acting control unit takes effect after any load variation, the stator current assumes a new value tending to let the motor operate at or near a point of maximum torque.
As further explained in my prior patent, the point of maximum torque occurs at different slip frequencies depending on the magnitude of the stator current. As that current increases, the point of maximum torque and thus the optimum operating point of such an induction motor shifts toward the higher slip frequencies. A load of given magnitude, therefore, can be driven either at a lower slip frequency with a higher current or at a higher slip frequency with a lower current. The patented system does not include any means for determining wether the current drawn by the stator after restabilization has in fact the lowest value compatible with the existing load and the selected rotor speed.
A somewhat analogous situation exists in the case of a synchronous motor where, within a permissible range, a minimization of the operating current required to drive a given load is possible by changes in the supply voltage with resulting adjustment of the phase lag between the rotor and stator fields.
Commonly owned German published specification No. 28 17 163 describes a method of and a system for so operating an induction motor, driving a spindle of a textile machine, that variations in load cause only a minimum change in rotor speed. The described procedure involves a monitoring of speed changes resulting from predetermined load incrementations at different supply voltages. Thus the selected optimum voltage is that with which the incrementation of the load causes the least speed variation.
Another commonly owned published German specification, No. 29 39 090, describes a method of and a system for controlling the rotor speed of several parallel-connected induction motors in a manner designed to maintain an optimum power factor. This is accomplished by varying, in a predetermined manner, the ratio of supply voltage to stator frequency with changes in that frequency or in the load. As pointed out in the latter publication, the largest power factor does not usually coincide with the steepest slope of the torque/speed characteristic (which is the parameter to be maximized according to German specification No. 28 17 163) so that a compromise between the two values may be desirable.
Neither of these two German publications directly addresses the problem of minimizing the consumption of electrical energy in the operation of an induction motor by a reduction of its stator current. That problem arises especially when a motor designed to drive loads of widely varying magnitude--or a group of substantially identical motors operating in parallel--is used only at a fraction of its capacity in driving a smaller load, with resulting loss of efficiency.